1. Field of the Invention
The present invention relates to an epitaxial wafer for semiconductor light-emitting devices (light-emitting diodes, semiconductor lasers), and particularly, to an epitaxial wafer for semiconductor light-emitting devices suitable for aluminum gallium indium phosphorus (AlGaInP) series light-emitting devices using magnesium (Mg) as a p-type dopant, and a semiconductor light-emitting device fabricated using the same.
2. Description of the Related Art
Generally, conventional AlGaInP series crystal growth for semiconductor light-emitting devices using metal organic vapor phase epitaxy (MOVPE) uses silicon (Si) and selenium (Se) as n-type dopants, and zinc (Zn) and Mg as p-type dopants. In epitaxial wafers for semiconductor laser (LD) applications, when Zn is used as the p-type dopant, the carrier concentration of a p-type cladding layer is typically set to a relatively low concentration of the order of 4×1017 cm−3.
In recent years, in semiconductor lasers, high-density optical disk devices which use an AlGaInP series visible-ray semiconductor laser as a light source have been actively developed. A light source for read/write in this high-density optical disk device requires stable high-power and high-temperature operation. To this end, the carrier concentration of the p-type cladding layer has to be made still higher.
However, doping Zn to a high concentration would cause Zn to diffuse into an active layer during epitaxial growth, which would result in the problem of deterioration of device characteristics and reliability. For this reason, the doping concentration of Zn has to be low. More recently, as the p-type dopant, Mg with a small diffusion coefficient compared to that of Zn has been used for making the carrier concentration of the p-type cladding layer high.
Also, it is known that, in an n-type-upper-layer light-emitting diode (LED) in which on a p-type substrate are sequentially stacked a p-type cladding layer, an active layer, an n-type cladding layer and an n-type current diffusion layer, in order to solve the problem of deterioration of brightness caused by formation of a non-luminescent center resulting from Zn diffusion into the active layer where Zn is the p-type dopant of the gallium phosphorus (GaP) substrate, a zinc diffusion prevention layer is formed between the p-type cladding layer and the substrate, or in a portion of the p-type cladding layer (see, e.g., Japanese patent application laid-open No. 2002-111052).
Also, in a conventional epitaxial wafer for semiconductor light-emitting devices, for the purpose of forming a low-resistance electrode, in a top layer is formed a low-resistance cap layer (contact layer) doped to a high concentration. This cap layer is typically formed of gallium arsenic (GaAs), where Zn is used as a dopant so that it can be doped to a high concentration (see, e.g., Japanese patent application laid-open No. 9-69667).
It is known that, during high-power and high-temperature operation of the LED and LD, leak current due to electron overflow from the active layer to the p-type cladding layer becomes large, so that threshold current and operating current thereby increase. To achieve stable high-power and high-temperature operation, it is desirable that the carrier concentration of the p-type cladding layer is made as high as possible, but when Zn is used as the p-type dopant as in the conventional art, as the carrier concentration of the p-type cladding layer is made higher, Zn is caused to diffuse into the active layer, which results in a larger photoluminescence spectrum full width at half maximum (hereinafter, PL FWHM) of the active layer, and impaired crystal quality of the active layer, which becomes the cause of an increase in threshold current and operating current, and a reduction in reliability. As a solution thereto, the present applicant has suggested as a prior application that, using MOVPE as the crystal growing means, in an epitaxial wafer for LDs in which on an n-type GaAs substrate are sequentially stacked at least an n-type AlGaInP cladding layer, a multi-quantum well (MQW) active layer, a p-type AlGaInP first cladding layer, a p-type gallium indium phosphorus (GaInP) etch stop layer, a p-type AlGaInP second cladding layer, and a p-type gallium arsenic (GaAs) contact layer, the p-type dopant of the p-type AlGaInP first cladding layer, p-type GaInP etch stop layer and p-type AlGaInP second cladding layer is Mg; the p-type dopant of the p-type GaAs contact layer is Zn; and the carrier concentration of at least the p-type AlGaInP first cladding layer of the p-type AlGaInP first and second cladding layers is in a range of 8×1017 cm−3 to 0.3×1018 cm−3.
In this manner, it is effective to use Mg as the p-type dopant of the p-type AlGaInP cladding layer and Zn as the p-type dopant of the p-type GaAs contact layer which allows a carrier concentration of more than 1×1019 cm−3 to be relatively easily obtained so that sufficiently small contact resistance can be obtained. This allows the carrier concentration of the p-type cladding layer to be made higher to the order of 1×1018 cm−3.
As a problem, however, it has been found out that doping the carrier concentration of the p-type cladding layer to a higher concentration than 1×1018 cm−3 makes mutual diffusion of Zn and Mg remarkable, which results in the phenomenon of Zn of the p-type contact layer diffusing into the p-type cladding layer and active layer which are not at all doped with Zn. Therefore, there is the problem that further doping to a higher concentration than 1×1018 cm−3 results in a larger PL FWHM of the active layer, as in the case of use of Zn only as the p-type dopant.
In other words, in the prior art, when Zn which tends to diffuse is used as the p-type dopant of the p-type GaAs cap layer formed in the top layer, there is the problem that Zn doped into this p-type GaAs cap layer is caused to diffuse through the underlying p-type cladding layer into the active layer during its growth, which results in a deterioration in luminescent characteristics of the active layer.
As explained as the above prior application, in the case of use of Mg as the p-type dopant of the p-type cladding layer, particularly this Zn diffusion is remarkable, and in addition, diffusion of Mg which should originally be relatively difficult to diffuse is facilitated, which results in the significant problem of deterioration in luminescent characteristics of the active layer and the life of the device.